$(function(){

	var clientH =document.documentElement.clientHeight;
	$(window).bind('scroll',function(){
		var scrollTop = document.documentElement.scrollTop||document.body.scrollTop;
		if(scrollTop>=clientH){
			$('#backTop').css('display','block');
		}else{
			$('#backTop').css('display','none');
		}
		
		$('#backTop').tap(function(){
			document.documentElement.scrollTop = 0;
			document.body.scrollTop = 0;
		});
	});

});